Field of the Invention
The invention relates to a field device for determining at least one measured variable. The measured variable is, for example, the volume or mass flow, the density, the fill level, the temperature, the pH, or the electric conductivity of a medium in a tube, container or channel. The invention further relates to a method for communicating the presence of a state of a field device to a receiving device.
Description of Related Art
Field devices designed as measuring devices are used in modern process automation in order to monitor media or processes or to control them based on determined measured values.
Several standards are known for the output of measuring signals determined by the field devices. In addition to simple signal outlets, such as 4 . . . 20 mA current outlet, pulse, or frequency outlets, state outlets are also used for transmitting the device state or measured value state. Furthermore, there are more complex fieldbus systems, such as HART, Fieldbus Foundation or Profibus.
In addition to merely determining the measured variable, modern field devices are able to determine errors. Such errors consist, for example, of the occurrence of inadmissible measured variables (e.g. temperatures that are too high) or of the occurrence of a problem in the field device itself—e.g. in signal processing or in individual components of the field device. This possibility of self-diagnosis is especially significant in safety-critical applications.
Field devices are often developed according to IEC 61508 or, in particular, according to IEC 61511, derived especially for the processing industry for use of the field devices in particularly dangerous and safety-critical processes. Safety or SIL (safety integrity level) functions are thereby at least partially adopted in the field devices. The higher the safety integrity level of a system is, the lower the chance that the required safety function is not carried out by the system as needed. The field devices or components of the field devices developed according to the respective specifications are considered safe or are used for a safety function.
A so-called safety circuit consisting of sensors, actuators, transmitting elements, logic processors, etc., which are accordingly combined with one another for fulfilling safety requirements, is used for the safety function.
Examples of the safety function are the safe deactivation of a motor, the closing of a valve, or the safe deactivation of operation of a process system or at least a part of a system, in which errors have been detected.
Error states are signalized in field devices, for example, when using current outlets with 4 . . . 20 mA signals (these can be, for example, two or also four wire devices), in that current values outside the given range between 4 mA and 20 mA are issued.
If, for example, and error occurs, so that a safe functioning of the field device is no longer guaranteed, then the field device is transferred to a safe state, which can optionally consist of the device shutting itself down. Furthermore, depending on the use, the error current can be evaluated in a SIL application, and the system or at least the parts of the system connected to the field device can be transferred into a safe state for fulfilling the safety function.
According to the prior art, the disadvantage of this error signal is that a measured value from the field device is no longer provided. In some situations, however, it would be advantageous for an evaluation or, respectively, for the taking of measures to be able to access this measured value.